The present disclosure relates to thermally actuated flow-control valves. More particularly, the present disclosure relates to valves including wax-filled actuators employed to control the flow of coolant to heat producing components in engine systems.
The wax actuator conventionally comprises a rigid housing surrounding a reservoir filled with wax formulated to transition from solid to liquid in response to a predictable increase in temperature. The housing is made of strong, thermally conductive material, such as brass, although other materials may be suitable. A piston is arranged to reciprocate in a guide that defines a bore in communication with the reservoir. A flexible diaphragm, plug, or other seal arrangement may be disposed between the wax and the piston to contain the wax in the reservoir. The wax expands in volume as the wax becomes a liquid, generating a force that is directed into the bore of the guide, and pushes the piston away from the reservoir. Thus, the axial length of the actuator changes according to the temperature of the wax, which is responsive to the temperature of the surrounding environment. Wax-filled actuators are reliable temperature sensitive actuators that require no external energy, such as electricity and are therefore self-contained.
Many systems are designed to operate within a specific temperature range and are equipped with heat exchange assemblies to add or remove heat to maintain the correct operating temperature. It is common to circulate fluid through such systems as a means of transmitting heat from one location to another. For example, it is common to circulate transmission fluid in a motor vehicle transmission through a radiator external to the transmission to remove heat from the transmission to prevent overheating during operation. However, it is also important for the transmission to quickly reach and maintain a minimum operating temperature. Therefore, it is common to equip the transmission with a temperature sensitive valve to alter the flow path of the transmission fluid depending upon the temperature of the fluid. The fluid will have a path bypassing the radiator at fluid temperatures below the minimum operating temperature, and a flow path through the radiator at temperatures approaching a maximum operating temperature. A wax-filled actuator may be employed to move a valve member between a cold position bypassing the radiator and a hot position circulating fluid through the radiator in response to the temperature of the fluid.
Such systems may require the valve member to move in response to fluid pressure regardless of the length of the actuator. It is common to employ a spring or bias member associated with the valve member to allow the valve member to move independently of the actuator piston in situations where pressure relief is required. Wax-filled actuators continue to extend in response to increased temperature, so it may also be necessary to accommodate over-extension of the actuator to prevent damage to the actuator or surrounding structures.
The wax-filled actuator is typically positioned in a housing or aperture filled with the fluid, with variable axial length of the actuator employed to move a valve member to alter the flow path of the fluid. A return spring is positioned to return the piston and valve to the retracted/cold position when the temperature of the fluid falls and the wax returns to its smaller volume. The return spring is selected to overcome the friction of the piston in the axial passage and any linkage or valve associated with the actuator, to ensure reliable return to the retracted/cold position.
While wax-filled actuators have gained wide acceptance in temperature control systems, it can be difficult to configure an actuator and valve to provide adequate valve movement and pressure relief in a compact configuration. This is especially difficult when a relief bias member is incorporated into the control valve that accommodates over-extension of the actuator at temperatures above the normal operating range for the system.
Consequently there exists a need for a simple, compact, and accurate thermally actuated flow-control valve that incorporates pressure relief and over-temperature relief.